Texture matters: The role of coatings in extruded pet food
New technological advances, such as dual layering for coated kibble, show a sustained commitment to innovative pet nutrition.
The coating of kibble is a key evolution in extruded food to increase palatability. It is also an important step in protecting the kibble core from oxidation, moisture and breakage while providing a means of adding functional ingredients such as probiotics to a pet food.
Factors affecting texture and coating
Coating layers provide scope for functional ingredient inclusion externally and creating various textures, as well as offering palatability options.
Several factors affect the coating and texture of extruded pet foods, and as a result the palatability. The quantity of fat that can be successfully applied as an external coating varies according to the method with which it is applied, the texture and porosity of the kibble, and the type of fat – to name just a few factors.
Vegetable and fish oils are predominantly polyunsaturated and therefore liquid at room temperature. Poultry fat tends to be liquid to semi-solid at room temperature, while pork grease or lard, ruminant tallows and coconut oil are solid at room temperature due to their higher degree of saturation.
The application of solid fats necessitates the warming of these oils prior to external application, as a liquid format is required. The appearance and textures created by each of these coating options also differs.
It is also important to consider the quantity of liquid applied to kibble externally. Less than 2% fat or liquid palatant coating can mean that kibble appears anemic or that the coating is uneven.
If the coating is too great, the kibble can have a greasy texture with the possibility of fat transferring to the packaging or even pooling in the bag, especially in warmer conditions.
Methods of application
Within the area of extruded products, the wide diversity in foods requires different approaches for coating. Methods such as batch coating and continuous drum coating are now considered more traditional methods.
Newer approaches suit different products, depending on the density, porosity and quantity of external coating and the intended consumer.
Continuous paddle coating and vacuum coating provide more specialized solutions, uniformity of coating or precision. Atmospheric coating methods usually involve a tumble drum spray or falling curtain system, which can work as a continuous process. Where a liquid palatant or digest is used, this may also be applied with the fat coating.
Vacuum batch coating is a solution to draw more externally applied fat into a kibble, allowing more fat to be applied externally than with atmospheric coating. This is especially beneficial when there is a larger quantity of fat to be applied externally, or for denser, less porose kibble where fat or liquid coatings aren’t easily absorbed.
Vacuum systems can work at high, medium or low settings, with larger quantities of external fat and denser kibble requiring higher vacuum settings to achieve the desired results. A recent study by Symrise Pet Food (SPF) found that vacuum coating of kibble improved palatability in cats.
Differential cooling leaves time between fat coating and the following dry ingredient coating. Often, powder ingredient coating occurs directly after liquid coating, to better allow the powder to adhere to the kibble.
A pause, however, of for example 24 hours, allows the fat coating to reach solid state, prior to the powder coating stage. The reason for this is to retain dry palatants at the kibble surface rather than becoming embedded in the fat coating.
The SPF palatability study also found that this technique significantly improved palatability for cats when compared with atmospheric coating.
Nutritional possibilities
As innovations and trends within the pet food industry evolve, there are both opportunities and challenges for coating and texture. Coating technology is advancing, and the process of liquid, followed by powder coating gives opportunity for novel ingredients to be incorporated into the coating layer of kibble, rather than being limited to the core.
As the functional nutrition trend continues, the scope for ingredients to be included post-extrusion is exciting. Future advances in kibble technologies may further differentiate between the core and coating layers, to optimize the nutrition and characteristics of each when developing and finessing pet foods.
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